Only sun-lit leaves of the uppermost canopy exceed both air temperature and photosynthetic thermal optima in a wet tropical forest
Only sun-lit leaves of the uppermost canopy exceed both air temperature and photosynthetic thermal optima in a wet tropical forest
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DOI：
10.1016/j.agrformet.2021.108347
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发表时间：
2021-02-06
影响因子：
6.2
通讯作者：
Cavaleri, Molly A.
中科院分区：
文献类型：
--
作者：
Miller, Benjamin D.;Carter, Kelsey R.;Cavaleri, Molly A.
热带森林是在相对狭窄的温度范围内形成的，因此可能比高纬度的森林更容易受到气候变化的影响。最近的证据表明，低地热带森林的冠层可能已经超过光合作用的最高温度。高度可以强烈影响森林冠层树叶的小气候和生理，但在热带森林中很少研究冠层微气象学的垂直趋势。为了提高我们的理解如何气候和微气象条件影响热带树木的功能，我们评估了垂直梯度的光合光子通量密度，蒸汽压赤字，空气温度，叶温，叶和空气温度之间的差异（Δ T）在波多黎各热带潮湿的森林。气温和水汽压亏缺均随高度线性增加。然而，在0-16米的阴影树叶中，叶温没有显着差异，而最上层（20米）比其他树叶高出4摄氏度，在最高辐射强度下比空气温度高出5摄氏度。结果，在遮荫的中间冠层和林下的叶片温度显示出几乎变温的行为（即，叶片温度=空气温度），而最上层的冠层表现出高温行为（即，叶片温度大于空气温度），揭示了不同的温度调节策略，阳光照射与阴影的树叶。此外，最上层冠层是唯一一层超过平均光合温度最佳为这个网站（T-opt = 30.2 +/- 1.1摄氏度）。由于上层树冠在整个森林的光合作用中起着不成比例的重要作用，持续变暖可能会削弱热带地区的碳汇能力。然而，遮荫的叶子可能能够随着进一步变暖而增加碳的吸收，因为它们似乎能够保持温度低于光合作用最佳值，可能是在最上层冠层提供的辐射屏蔽的帮助下。
Tropical forests have evolved under relatively narrow temperature regimes, and therefore may be more susceptible to climatic change than forests in higher latitudes. Recent evidence shows that lowland tropical forest canopies may already be exceeding thermal maxima for photosynthesis. Height can strongly influence both the microclimate and physiology of forest canopy foliage, yet vertical trends in canopy micrometeorology are rarely examined in tropical forests. To improve our understanding of how climatological and micrometeorological conditions affect tropical tree function, we assessed vertical gradients of photosynthetic photon flux density, vapor pressure deficit, air temperature, leaf temperature, and the difference between leaf and air temperature (Delta T) in a Puerto Rican tropical wet forest. Both air temperature and vapor pressure deficit increased linearly with height. Leaf temperature, however, did not significantly differ across the shaded foliage from 0-16 m, while the uppermost layer (20 m) was up to 4 degrees C hotter than the rest of the foliage and up to 5 degrees C hotter than air temperature at the highest radiation intensity. As a result, leaf temperatures in the shaded middle canopy and understory showed nearly poikilothermic behavior (i.e., leaf temperatures = air temperature), while the uppermost canopy strata showed megathermic behavior (i.e., leaf temperatures greater than air temperature), revealing different thermoregulation strategies for sun-lit versus shaded foliage. In addition, the uppermost canopy was the only stratum to exceed mean photosynthetic temperature optima for this site (T-opt = 30.2 +/- 1.1 degrees C). Because the upper canopy plays a disproportionately large role in whole-forest photosynthesis, continued warming could potentially weaken the tropics' carbon sink capacity. However, the shaded leaves may be able increase carbon uptake with further warming because they appear to be able to maintain temperatures below photosynthetic optima, possibly with the help of radiation shielding provided by the uppermost canopy layer.